Mixing device for introducing additives into a liquid

ABSTRACT

A mixing device for introducing additives, particularly gaseous additives into a liquid to produce mixtures, solutions, emulsions or suspensions, comprising a rotatable drive shaft immersible in the liquid, a rotatable mixing propeller driven by the drive shaft, a bore in the drive shaft and/or a separate carrier tube or tubes concentrically surrounding the drive shaft, through which the additives can be delivered and a stationary guide ring concentrically surrounding the mixing propeller at a radial distance from its outer edge.

United States Patent Inventor Appl. No,

Filed Patented Assignee Priority MIXING DEVICE FOR INTRODUCING ADDITIVES Hubert Fuchs Mayen, Germany Sept. 26, 1968 June 15, 1971 The De Laval Separator Company Poughkeepsie, N .Y.

Sept. 26, 1967 Austria INTO A LIQUID 22 Claims, 18 Drawing Figs.

U.S.Cl

Int. Cl.

Field of Search References Cited UNITED STATES PATENTS 6/1930 Wait 2,488,447 1 1/1949 Tangen et a1. 259/8X 2,706,622 4/1955 Staaf 259/96 2,769,623 11/1956 Cawood 259/134 2,801,083 7/1957 Balassa 259/24X 2,953,359 9/1960 Mau 259/8 3,204,768 9/1965 Daniel 259/7X 3,334,869 8/1967 Mukai 259/23X 3,355,106 ll/l967 Graham. 259/96X 3,374,490 3/1968 Carlson 259/8X 3,457,047 7/ 1969 Tokimatsu et a1. 259/24X Primary ExaminerEdward L. Roberts Altorney- McGlew and Toren PATENTEDJUNISISH 3,5 4, 40

' sum 1 BF 4 FIG.5

INVENTOR.

HUBERT FUCHS A++owmu s PATENTEU JUN] 5 19?:

sum 2 OF 4 INVENTOR. H UBE RT FU CH 5 BY wwmi 171w A++orneg s PATENTED JUN} 5 |97l SHEET 3 OF 4 INVENTOR. H U BERT F'U CH 5 A'H'or-negs MIXING DEVICE FOR INTRODUCING ADDITIVES INTO A LIQUID The invention concerns a mixing device for introducing additives-for example, powdery, fluid, gaseous, inorganic or also organic substances-as secondary components into liquids in order to produce mixtures, solutions, emulsions or suspensions.

Such a mixing device may be used to carry out physical operations, chemical reactions or biological processes; and are particularly intended for the biological purification of water in large quantities in collector tanks, storage units, reservoirs for water supply, filter beds or open water (sea) to assist the biological self-purification of such water.

lt is known that flowing water has a relatively high biological self-purification action as the organic substances and particles which still remain suspended therein at given average speeds of flow can easily take up the molecular oxygen necessary for their decomposition and on the other hand molecular oxygen is also quickly replaced from the fresh air above the surface of the water. However, in substantially stationary water, i.e. stagnant or quite slowly flowing water, the substances contained therein in the form of floating materials or sedimentary materials in suspension are extensively deposited until this so-called relative sedimentation" even reaches almost a saturation value; with some of the organic particles still decomposing as they sink. Thus, there firstly results an oxygen deficit in the upper layers of the water and finally a state of almost complete oxygen loss and formation of sludge.

According to the invention there is provided a mixing device for introducing additives into a liquid to produce mixtures, solutions, emulsions or suspensions, comprising a rotatable drive shaft immersible in the liquid, a rotatable mixing propeller driven by the drive shaft and/or a separate carrier tube, surrounding the drive shaft, through which the additives can be delivered, and a stationary guide ring concentrically surrounding the mixing propeller at a radial distance from its outer edge.

In this mixing device the stationary guide ring itself can in the simplest embodiment be substantially in the shape of a simple truncated cone surface, to the inner edge of which a hollow throat is preferably fitted. In a particularly advantageous embodiment the guide ring is substantially bellshaped or bowl-shaped. Irrespective of its basic shape, the guide ring can also be substantially in the shape of an aerofoil.

In a further advantageous embodiment of the mixing device both the hollow drive shaft of the mixing propeller and also the carrier tube enclosing the drive shaft are used to deliver two separate additives which are fed to these tubes through lateral apertures.

The mixing device can be inserted substantially vertically downwardly into the liquid to be treated, while the discharge head of the delivery tubes and the stirrer proper, i.e. the rotating mixing propeller with the stationary guide ring, are to be located at a sufficient distance above the bottom of the container or water in question. When the mixing propeller is rotated the liquid is thrown continuously radially outwardly and there is formed at its center a suction area into which the various additives to be introduced are sucked relatively quickly through the hollow drive shaft or through the concentric stationary delivery tubes, and are intensively mixed into the radially outwardly flowing liquid. The stationary guide ring concentrically surrounding the rotating propeller particularly controls in an advantageous manner the circulation of the flow of liquid and considerably intensifies the mixing of the liquid with the additives or its range of operation in the body of liquid is substantially widened.

The mixing propeller itself which is used in the mixing device can be with two or more blades and its individual blades are preferably substantially spiral-shaped, for example corresponding to a function illustrated in polar coordinates havingthe general form r=g+b wherein a and b are constants the basic shape of the propeller blades can also correspond to an exponential function illustrated in polar coordinates;r=a +b or r=e +b,wherein a and b are general constants and e is the base of natural logarithms.

in a preferred embodiment of the mixing device the constructions of the multibladed mixing propeller is such that a substantially horizontal guide web or a plurality of such guide webs is/are inserted between each adjacent spiral-shaped blades, at the underside of which web/webs the liquid to be treated which flows into the suction area formed, and at the topside of which web/webs, the axially delivered additives are conducted radially away from the mixing propeller with as little turbulence as possible.

In simple mixing devices, i.e. mixing devices for delivering only one additive, particularly through the stationary carrier tube arranged concentrically to the drive shaft for the propeller, the arrangement is such that one horizontal guide web is provided on the mixing propeller between each two blades adjacent to the hub of said propeller, which guide web tapers wedge-shaped toward its outer edge and is preferably concavely rounded on both sides and which is preferably extended further outwardly at the concave suction side (i.e. the inside) of the first blade in the direction of rotation, than at the convex pressure side (outside) of the following blade.

For mixing devices having two or more stationary delivery tubes arranged concentrically to the drive shaft for the mixing propeller, which are therefore used for a corresponding number of additives, there are two specific embodiments of the mixing propeller and of the discharge head of the delivery tubes, ensuring that the separate additives flow into the circu lating liquid with as little hindrance and difficulty as possible, thereby ensuring a higher degree of effectiveness for the desired mixing efficiency.

In one embodiment of this mixing device the stationary concentric delivery tubes are preferably flared in a trumpet-shape at the discharge end and there are provided on the mixing propeller between adjacent blades at the upper edge thereof and aligning with each of the two delivery tubes a respective scoop-shaped guide web ending in a horizontal portion, and also at a spacing therebelow, there is provided directly on the hub of the propeller a radially directed guide web which is concave on both sides. In another embodiment this mixing device for a plurality of additives is of such a construction that the concentric delivery tubes are stepped relative to each other at the discharge end, the steps rising outwardly, and the propeller blades are also correspondingly stepped at their upper edges, ie they engage over the open ends of the tubes, while between adjacent propeller blades there are disposed respective substantially horizontal guide webs which adjoin the discharge ends of each delivery tube and are correspondingly spaced-apart, and a further guide web is mounted directly on the hub of the propeller at a spacing below each of said substantially horizontal guide webs.

Finally, mixing devices with a hollow drive shaft for delivering the particular additive or an additional additive can advantageously be so constructed that radial discharge passages are arranged in the hub of the mixing propeller and are connected to the central bore of the hollow shaft, opening between adjacent propeller blades, preferably substantially at the level of the horizontal main guide web.

The invention is diagrammatically illustrated by way of example in the accompanying drawings, in which:

FIG. 1 shows the device immersed i'n the operative positioh in a liquid;

FIGS i to 5" show the iawer'seirarartre mar arine?" FIG. 11 shows another embodiment of this shaft bearing in an offset vertical cross section along a line at a position similar to the position of line XI-XI of FIG.

FIG. 12 shows a plan view of a spiral-shaped mixing propeller; I

FIG. 13 shows a part sectional elevation through an embodiment ofa mixing propeller;

FIG. 14 is a plan view of the propeller shown in FIG. 13;

FIG. 15 shows a sectional detail taken on line XV-XV of FIG. 14;

FIG. 16 shows an embodiment of a propeller in a direction indicated by arrow XVI on FIG. 14;

FIGS. 17 and I8 show further embodiments of a propeller.

Referring to the drawings FIG. 1 shows the basic construction of the complete mixing device inserted with the stirrer proper into a liquid F, i.e. the mixing device is in its operating position. A shaft 2, driven by means of a motor I, is mounted in a vertical carrier tube 3, fixed to the housing of the motor 1, and carries at its lower end a mixing propeller 5. Arranged within the carrier tube 3 or also in the drive shaft 2 itself is a downwardly open delivery passage for a secondary component A to be introduced into the liquid F. A plurality of such delivery passages can be provided if required. The delivery passage is connected to a corresponding delivery pipe 32 including a regulating valve 33 and the mixing propeller 5 is concentrically surrounded at a radial spacing from its outer edge by a stationary guide ring 60 for guiding the flow of liquid which passes by the propeller 5. The guide ring 60 is in the form of a truncated cone surface in this embodiment and the inner edge of the ring lies above the level of the plane of the propeller.

FIGS. 2 to 5 show four particularly advantageous embodi-- ments of the guide ring which concentrically surrounds the rotating mixing propeller 5. In the embodiment of FIG. 2, a guide ring 61 is substantially in the form of a truncated cone surface but includes at its inner edge a collar 61' forming a passage. In the embodiment of FIG. 3, a guide ring 62 is substantially in the shape of an aerofoil, tapering toward its outer edge. FIGS. 4 and 5 show bell-shaped suspended guide rings 63 and 64 respectively which are particularly adapted for adjacent flow of liquid circulation. However, the guide ring enclosing the mixing propeller 5 can be of any cross-sectional shape which is desirable from the point of view of flow, e.g. paraboloidal or hyperboloidal; all these various, upwardly or downwardly inclined shapes can obviously also be in the form of an aerofoil, as shown in FIG. 3.

In all the types of guide ring 6-64 the angle of inclination or of incidence of the ring can be positive or negative end depends in individual cases both on the viscosity of the liquid F to be treated and also its the ratio between the amounts or concentration of the secondary components to be introduced into said liquid. In every case the specific profile and position of the guide ring 60 to 64, and particularly the beadlike curved construction of its inner edge as shown in FIG. 2 or FIG. 3 are intended to adapt this profile in the best possible manner to the circulation flow caused by the rotating mixing propeller in the liquid F and as a further effect should ensure that this flow is spread radially as far as possible; this then ensures that the secondary components introduced can be mixed intensively and over a long period in as large a region as possible of the liquid F, providing a higher degree of efficiency.

In the mixing device illustrated in FIG. 6, which for simplicity is shown without a guide ring, the secondary component A is introduced in known manner into the region of the mixing propeller through a carrier tube 30 surrounding the solid drive shaft 2. A shaft bearing 36 fitted at the bottom of the carrier tube 30 must be provided with suitable apertures for secondary component A which is sucked into a central suction area by the mixing propeller 5 when the device is in operation.

FIG. 7 shows a basic arrangement of a mixing device for introducing two secondary components A and B through two concentric delivery tubes 20 and 30 with the inner tube 20 also serving as the drive shaft 20 for the mixing propeller 5.

The secondary component A is supplied to the hollow drive shaft 20 through lateral orifices 24 which lie between two sealing rings 25 fitted in the concentric carrier tube 30 on either side of a delivery pipe 22. In this embodiment the secondary component A, sucked in by the mixing propeller 5 through the tube 20, passes to the suction area through the lower open end of the tube 20, and the lower shaft bearing 36 mounted in the outer delivery tube 30 is, as in the embodiment of FIG. 6, provided with apertures for the secondary component B, which is fed to the carrier tube 30 through the pipe 32 and an aperture 34 in the wall of the carrier tube 30.

The mixing device shown in FIG. 8 is given as a general example of a mixing device for introducing three different secondary components A, B and C into a liquid, accordingly having three concentric delivery tubes 20, 30 and '40. As in the embodiment of FIG. 7 the drive shaft 20 is again in the form of a hollow shaft, and the secondary component A is introduced through lateral apertures 24 into the hollow shaft 20 through the pipe 22 connnected to the carrier tube 30 between the sealing rings 25. In addition the carrier tube 30 which is also connected to the delivery pipe 32 is used to deliver the second secondary component B in a manner similar to that illustrated in FIGS. 6 and 7; finally a second stationary delivery tube 40 is disposed concentrically around said carrier tube 30, through which delivery tube 40 the third secondary component C, supplied through a lateral pipe 42 and aperture 44 in the pipe 40, is fed in the same manner. The two delivery pipes 22 and 32 connected to the carrier tube 30 for the secondary components A and B are separated from each other by the lower sealing ring 25 and the outer delivery tube 40 can be open downwardly. In this arrangement all three secondary components A, B and C are sucked into the central suction zone by the rotating mixing propeller 5 and are mixed into the liquid F which flows radially outwardly under pressure.

In these mixing devices having a plurality of vertical stationary delivery tubes 20, 30 and 40 for a corresponding number of secondary components A, B and C, which can if required be delivered thereto under pressure, regulating members, as shown at 33 in the embodiment of FIG. 1, are positioned in the delivery pipes 22, 32 and 42 to said various delivery tubes 20, 30 and 40 to meter the additives which are to be introduced into the liquid. These regulating members 33, e.g. magnetic valves, can desirably also be automatically controlled by electronic sensors which are disposed in the liquid F to be treated. For example, the regulating members 33 can be automatically controlled as a function of the concentration of the secondary component in question, i.e. of the 0 content of the water. The concentric delivery tubes 20, 30 and 40, for the various secondary components can be in communication one with another or possibly the outer delivery tube 30 or 40 can also be in communication with the surrounding liquid F, by means of small orifices, thus permitting the individual additives to be premixed if this is desired.

FIGS. 9 and 10 illustrate, for the embodiment of the mixing device of FIGS. 6 to 8, an enlarged view of the shaft bearing 36 for the drive shaft 2 or 20, which is fitted at the lower end of the stationary delivery tube 30. The body of the bearing 36 is provided with vertical apertures 37, here illustrated as simple holes, through which the particular secondary component A or B in question is sucked out of the delivery tube 30 by the action of the mixing propeller 5. Depending on the production type of bearing 36 and/or depending on the nature or viscosity of the secondary components to be mixed in, the apertures 37 can be of other desirable cross-sectional shape or aperture widths. The chosen widths of the apertures 37 will depend on the desired flow rate of the medium to be introduced; thus these apertures 37 can for example be in the form of sectorshaped recesses at suitable spacings from each other.

FIG. 11 shows a lower shaft bearing 36 with a particularly advantageous arrangement of the orifices 38 for the secondary component which in this embodiment are directed in the direction of rotation at an angle to the blades of the mixing propeller 5, ensuring that the secondary component is intensively mixed into the main flow of the circulating liquid.

FIG. 12 shows a multibladed mixing propeller 5 for the stirrer of the mixing device, the propeller blades 5 of which are substantially spiral-shaped, being constructed in this embodiment in a particularly advantageous manner according to an exponential function expressed in polar coordinates r--a/-,l1 +b, r=a +b or r=e"+b, where e is the base of natural logarithms anda and b are cohstants. When mixing propellers with blades with an exponential curve of this type are used, the mixing effect or efficiency achieved with respect to the motor output is particularly high, due to the high volume of liquid embraced by the mixing action.

FIGS. 13, 14 and 15 illustrate another particularly advantageous embodiment of the mixing device in which in the simplest embodiment only one secondary component A is delivered to a mixing propeller 50, through the stationary carrier tube 30 which is concentric to the drive shaft 2 and which is flared like the mouth of a trumpet at a discharge head 30'. The shaft bearing 36 fitted at the lower end of this delivery tube 30 is again provided with apertures 37 for the discharging medium A. The mixing propeller 50 is fixed by a hub 51 to a shaft journal 2' and is provided with spirally curved trailing blades 52, there being for example three blades in this embodiment. A horizontal guide web 55 is provided between adjacent blades 52, and is joined to the hub 51; as shown in the vertical cross section of FIG. 15 taken along the line XVXV of FIG. 14. The guide web 55 is tapered wedgelike toward its free outer edge and is desirably concavely rounded on both of its sides 55' and 55"In in addition, as shown in FIG. 14, the guide web 55 is preferably extended further outwardly at the concave suction side (i.e. at the inside) of the first blade in the direction of rotation, than at the convex pressure side (outside) of the subsequent blade. I

FIG. 16 shows an advantageous embodiment of a mixing propeller to which are supplied secondary components A and B, one secondary component (A) again being delivered through the stationary carrier tube 30and the other secondary component (B) through the hollow drive shaft 20 of the mixing propeller 50. A central bore 21 of the drive shaft 20 extends into a shaft journal 20 on which is fitted the mixing propeller 50 with its hub 51 the propeller hub 51 is provided with horizontal radial discharge passages 53 which communicate with the central bore 21 of the shaft journal 20' and open out between adjacent propeller blades 52, desirably at a position 54 lying substantially at the level of the horizontal guide web 55. Therefore when the mixing device is operated the second secondary component B discharges out of the hub 51 of the mixing propeller approximately halfway up the blades 52 thereof, whereby it is introduced between the two flow currents of the circulating liquid F which are deflected radially by the horizontal guide webs 55, and the first secondary component A which is delivered downwardly through the carrier tube 30; this arrangement provides that all three com ponents, F, A and B, are particularly intimately mixed.

FIG. 17 shows an embodiment of a mixing device in which two secondary components A and C are delivered through the two stationary delivery tubes 30 and 40 arranged concentrically with the drive shaft 2. The two concentric delivery tubes 30 and 40 are again flared at the discharge end at 30 and 40' respectively and in addition scoop-shaped guide webs 56 and 57 which end in a horizontal portion are mounted on the mixing propeller 50 between adjacent blades 52 at the upper edge thereof in alignment with a respective one of the two delivery tubes 30 and 40, i.e. in continuous extension of their surface line. At a distance below the guide webs 56 and 57, and in the same manner as in the embodiment described with reference to FIGS. 13 to 15, a radially directed guide web 55, which is concave on both sides, is joined directly to the hub 51 of the propeller. The particularly favorable effect achieved by this arrangement for optimum mixing of the circulating liquid F with the two secondary components A and C which are introduced between the two oppositely directed flow currents of the liquid F, is readily apparent from the arrows indicating the flow in FIG. 17.

FIG. 18 illustrates another embodiment of a mixing device with two concentric delivery tubes for two secondary com ponents A and C; in which the two delivery tubes 30 and 40 which are concentric to the drive shaft 2 are stepped relative to each other at the discharge end 30" and 40" respectively, the steps rising outwardly. On a mixing propeller 70 propeller blades 72 are provided at their upper edges with corresponding steps 73 and 74, which engage over the open ends 30" and 40", while between adjacent propeller blades 72, adjoining the discharge end 30" and 40" respectively of each delivery tube 30 and 40 and at suitable spacings from each other, are substantially horizontal guide webs 76 and 77. At a spacing below the guide webs 76 and 77 a further guide web 75 is mounted directly on a hub 71 of the propeller.

In all the mixing propellers 5, 50 or 70 hereinbefore described and used in the mixing device, the individual propeller blades 5', 52 or 72 can also be provided with apertures or can be indented at their outer edges or can be constructed to form fingers, thus achieving that the flow of liquid is subjected to an additional subdivided turbulence effect and is thereby mixed with the secondary components in a particularly intimate fonn, i.e. in the fonn of particularly fine bubbles.

The embodiments of the invention in which I claim: an exclusive property or privilege are defined as follows:

1. A mixing device particularly for adding a gas such as air to water for facilitating its purification, comprising a rotatable drive shaft immersible in the'liquid, a propeller affixed to said drive shaft and rotatable therewith and having blades shaped at the exterior bottom side to produce an upward whirling flow and a radial discharge of the liquid, a fluid carrier tube adjacent said shaft terminating in a discharge adjacent the top of said propeller, said propeller producing by the liquid flow and its configuration a negative pressure at the discharge of said carrier tube and an inflow of fluid through said carrier tube and out said discharge into the radial discharge of said liquid, and a stationary radially extending guide ring having an opening located adjacent said propeller and defining a generally radially extending surface surrounding the periphery of said propeller and located to deflect the fluid flow from said discharge and said liquid flow from said propeller outwardly so that it does not return directly upwardly to the liquid surface.

2. A mixing device as claimed in claim 1, in which the stationary guide ring surrounding the mixing propeller and spaced therefrom is inclined so that it is substantially funnelshaped with its inner edge at a level above the propeller.

3. A mixing device as claimed in claim 2, in which the stationary guide ring is in the form of a truncated cone.

' 4. A mixing device as claimed in claim 3, including a collar at the neck of the truncated cone.

5. A mixing device as claimed in claim 2, in which the guide ring is substantially inverted bell-shape.

6. A mixing device as claimed in claim 2, in which the guide ring has an aerofoil shape.

7. A mixing device as claimed in claim 1, in which the mixing propeller is multibladed having individual blades which are substantially spiral-shaped.

8. A mixing device as claimed in claim 7, in which the individual blades each correspond to a function expressed in polar coordinates having the general from r=a/tl1+b, wherein a and b are constants.

9. A mixing device as claimed in claim 7, in which the individual blades correspond in their basic shape to an exponential function expressed in polar coordinates: r=a' '+b or r=e'" +b, wherein a and b are general constants and e is the base of natural logarithms.

10. A mixing device as claimed in claim 7, including a substantially horizontal guide web disposed, on the multibladed mixing propeller between adjacent spiral-shaped blades, the liquid to be treated flowing into a suction area formed at the underside of the guide web .and the additives being delivered axially to the topside of the propeller and being drawn off radially from the mixing propeller with as little turbulence as possible,

11. A mixing device as claimed in claim 10, in which the horizontal guide web tapers wedgelike towards its outer edge, is concavely rounded on both sides and is extended further outwardly at the concave suction side (i.e. the inside) of one blade in the direction of rotation, than at the convex pressure side (i.e. the outside) of the following blade.

12. A mixing device as claimed in claim 7, in which the individual blades of the mixing propeller each gaper towards the outer edge.

13. A mixing device as claimed in claim 1, in which said carrier tube defining the passageway is a bore in the drive shaft of the mixing propeller and the drive shaft has a lateral orifice therein through which the additive can be delivered.

14. A mixing device as claimed in claim 1, said propeller having blades, and radial discharge passages arranged in the hub of the mixing propellers, connected to the bore in the hollow drive shaft and opening between adjacent propeller blades 15. A mixing device as claimed in claim 1, in which the means defining the passageway is a stationary tube concentric with the drive shaft and spaced therefrom to form an annular space between the drive shaft and the tube, the space being in communication with a delivery pipe for delivery of an additive.

16. A mixing device as claimed in claim 15, including a shaft bearing for the drive shaft in the end of the stationary carrier tube adjacent the mixing propeller, the shaft bearing including apertures for the flow of additives therethrough, said apertures being directed obliquely toward the blades of the mixing propeller in the direction of rotation of the propeller.

17. A mixing device as claimed in claim 15, including a flow control member located in the delivery pipe which supplies additive to the annular space to meter the additives to be introduced. I

18. A mixing device as claimed in claim 15, having at least two stationary delivery tubes arranged concentrically to the drive shaft, for supply of a corresponding number of additives,

the concentric delivery tubes being flared at the discharge end; said propellar having blades and scoop-shape guide webs on the mixing propeller between adjacent blades at the upper edge thereof, each in alignment with a respective one of the two delivery tubes, the scoop-shaped guide webs each ending in a horizontal portion; and radially directed guide webs at a distance below the guide webs, concave on both sides and mounted directly on the propeller hub.

19. A mixing device as claimed in claim 15, having at least two stationary delivery tubes concentric to the drive shaft for supply of a corresponding number of additives, the concentric delivery tubes being stepped relative to each other at the discharge end, the steps rising outwardly, said propeller having blades being correspondingly stepped at their upper edges to engage over the open ends of the tubes; a respective horizontal guide web between adjacent propeller blades, adjacent to the discharge end of each delivery tube and at corresponding distances from each other; and at a distance below said horizontal guide web, a further guide web joined directly to the hub of the propeller.

20. A method of adding a fluid particularly a gas such as air to a liquid such as water for facilitating its purification, comprising producing an upward liquid flow in the liquid below the surface thereof and then a radial outward flow of the liquid below the surface in a manner to create a respective pressure adjacent the radial outward flow, and permitting a fluid such as air to flow downwardly to the location of the radial outward flow under the influence of the pressure created by the radial outward flow, and deflecting both the incoming fluid and the liquid so that the flow remains in a general radial direction whereby to utilize the force of the gas and the liquid flow to move and to intermix the gas before the gas and the liquid flow is permitted to move the surface of the liquid.

21. A method, according to claim 20, wherein the gas and liquid flow is deflected substantially obliquely downwardly in res ect to the surface of the liquid.

2. A method, according to cla1m 20, wherein the gas and liquid flow is deflected substantially radially. 

2. A mixing device as claimed in claim 1, in which the stationary guide ring surrounding the mixing propeller and spaced therefrom is inclined so that it is substantially funnel-shaped with its inner edge at a level above the propeller.
 3. A mixing device as claimed in claim 2, in which the stationary guide ring is in the form of a truncated cone.
 4. A mixing device as claimed in claim 3, including a collar at the neck of the truncated cone.
 5. A mixing device as claimed in claim 2, in which the guide ring is substantially inverted bell-shape.
 6. A mixing device as claimed in claim 2, in which the guide ring has an aerofoil shape.
 7. A mixing device as claimed in claim 1, in which the mixing propeller is multibladed having individual blades which are substantially spiral-shaped.
 8. A mixing device as claimed in claim 7, in which the individual blades each correspond to a function expressed in polar coordinates having the general from r a/ psi +b, wherein a and b are constants.
 9. A mixing device as claimed in claim 7, in which the individual blades correspond in their basic shape to an exponential function expressed in polar coordinates: r a +b or r a +b, wherein a and b are general constants and e is the base of natural logarithms.
 10. A mixing device as claimed in claim 7, including a substantially horizontal guide web disposed, on the multibladed mixing propeller between adjacent spiral-shaped blades, the liquid to be treated flowing into a suction area formed at the underside of the guide web and the additives being delivered axially to the topside of the propeller and being drawn off radially from the mixing propeller with as little turbulence as possible.
 11. A mixing device as claimed in claim 10, in which the horizontal guide web tapers wedgelike towards its outer edge, is concavely rounded on both sides and is extended further outwardly at the concave suction side (i.e. the inside) of one blade in the direction of rotation, than at the convex pressure side (i.e. the outside) of the following blade.
 12. A mixing device as claimed in claim 7, in which the individual blades of the mixing propeller each gaper towards the outer edge.
 13. A mixing device as claimed in claim 1, in which said carrier tube defining the passageway is a bore in the drive shaft of the mixing propeller and the drive shaft has a lateral orifice therein through which the additive can be delivered.
 14. A mixing device as claimed in claim 1, said propeller having Blades, and radial discharge passages arranged in the hub of the mixing propellers, connected to the bore in the hollow drive shaft and opening between adjacent propeller blades
 15. A mixing device as claimed in claim 1, in which the means defining the passageway is a stationary tube concentric with the drive shaft and spaced therefrom to form an annular space between the drive shaft and the tube, the space being in communication with a delivery pipe for delivery of an additive.
 16. A mixing device as claimed in claim 15, including a shaft bearing for the drive shaft in the end of the stationary carrier tube adjacent the mixing propeller, the shaft bearing including apertures for the flow of additives therethrough, said apertures being directed obliquely toward the blades of the mixing propeller in the direction of rotation of the propeller.
 17. A mixing device as claimed in claim 15, including a flow control member located in the delivery pipe which supplies additive to the annular space to meter the additives to be introduced.
 18. A mixing device as claimed in claim 15, having at least two stationary delivery tubes arranged concentrically to the drive shaft, for supply of a corresponding number of additives, the concentric delivery tubes being flared at the discharge end; said propellar having blades and scoop-shape guide webs on the mixing propeller between adjacent blades at the upper edge thereof, each in alignment with a respective one of the two delivery tubes, the scoop-shaped guide webs each ending in a horizontal portion; and radially directed guide webs at a distance below the guide webs, concave on both sides and mounted directly on the propeller hub.
 19. A mixing device as claimed in claim 15, having at least two stationary delivery tubes concentric to the drive shaft for supply of a corresponding number of additives, the concentric delivery tubes being stepped relative to each other at the discharge end, the steps rising outwardly, said propeller having blades being correspondingly stepped at their upper edges to engage over the open ends of the tubes; a respective horizontal guide web between adjacent propeller blades, adjacent to the discharge end of each delivery tube and at corresponding distances from each other; and at a distance below said horizontal guide web, a further guide web joined directly to the hub of the propeller.
 20. A method of adding a fluid particularly a gas such as air to a liquid such as water for facilitating its purification, comprising producing an upward liquid flow in the liquid below the surface thereof and then a radial outward flow of the liquid below the surface in a manner to create a respective pressure adjacent the radial outward flow, and permitting a fluid such as air to flow downwardly to the location of the radial outward flow under the influence of the pressure created by the radial outward flow, and deflecting both the incoming fluid and the liquid so that the flow remains in a general radial direction whereby to utilize the force of the gas and the liquid flow to move and to intermix the gas before the gas and the liquid flow is permitted to move the surface of the liquid.
 21. A method, according to claim 20, wherein the gas and liquid flow is deflected substantially obliquely downwardly in respect to the surface of the liquid.
 22. A method, according to claim 20, wherein the gas and liquid flow is deflected substantially radially. 